The present invention pertains to monitoring apparatus that detect the state of contamination of optical components of an optical system (e.g., microscope, analysis device, or microlithography apparatus) that utilizes electromagnetic radiation such as X-rays or ultraviolet light, or a charged particle beam. The invention also pertains to optical systems (e.g., microscope, analysis device, or microlithography apparatus) including such a monitoring apparatus. The invention also pertains to microlithography methods including contaminant-monitoring of certain optical components.
X-rays produced by synchrotron radiation have high brightness and variable wavelength, and hence are used as X-ray sources for X-ray analysis devices, X-ray microscopes, and X-ray microlithography (projection-exposure) apparatus. Another useful X-ray source is a laser-plasma X-ray source (abbreviated xe2x80x9cLPXxe2x80x9d source). In an LPX source, a pulsed laser light beam is focused onto a target substance contained inside a vacuum chamber. The pulses of laser light impinging on the target substance create a plasma that emits X-rays. The X-rays radiating from the plasma are extracted and formed into an X-ray beam. LPX sources have a brightness comparable to that of synchrotron sources, and have the advantage of compactness. Consequently, LPX sources have been under intensive development recently as the X-ray source of choice for various applications.
Other X-ray sources that are attracting attention utilize a so-called xe2x80x9cZ-pinchxe2x80x9d plasma, dense plasma focus, or plasma created by a discharge in a capillary. These sources are relatively inexpensive.
In an X-ray microlithography apparatus, a reticle (defining a pattern) is irradiated by an X-ray beam from a source. After irradiating the reticle, the X-ray beam is manipulated and directed to form a corresponding image of the pattern on a suitable substrate (e.g., semiconductor wafer) previously xe2x80x9csensitizedxe2x80x9d with a coating of an appropriate xe2x80x9cresist.xe2x80x9d The X-ray beam is manipulated and directed using X-ray optical components (mainly specialized mirrors). The wavelength of X-rays used in conventional X-ray microlithography apparatus is in the extreme ultraviolet region, having a wavelength in the range of a few nanometers to approximately 50 nanometers. These X-rays are termed xe2x80x9csoftxe2x80x9d X-rays. Since many substances are highly absorptive to radiation in this range of wavelengths, adhesion of even a slight amount of a contaminant substance to an X-ray optical component can cause a conspicuous deterioration in the optical characteristics (e.g., reflectivity and transmissivity) of the X-ray optical component.
In optical devices that use soft X-rays, the optical path typically is evacuated to high vacuum to eliminate attenuation of the X-rays by the atmosphere. Accordingly, the X-ray optical components are enclosed in a vacuum chamber, and the interior of the vacuum chamber is evacuated to high vacuum using a suitable vacuum device such as a rotary-vane pump or diffusion pump, etc. Unfortunately, these vacuum devices tend to produce a slight back-flow of pump-oil vapor into the vacuum chamber during operation, thereby introducing oil molecules into the vacuum chamber. Also, the resist tends to outgas in a vacuum. With extended operation, these introduced oil molecules and resist-outgas molecules tend to accumulate on the X-ray optical components inside the vacuum chamber, causing progressive deterioration of the optical characteristics of the X-ray optical components.
In addition, LPX and discharge-plasma X-ray sources tend to produce particulate debris from the plasma and/or from structures located near the plasma. The debris can adhere to a nearby X-ray optical component, causing the optical characteristics of the optical component to deteriorate. Such deterioration can result in a decline in throughput of the apparatus itself.
In conventional microlithography apparatus, there currently is no practical technique with which to monitor the degree of contamination of the optical components during operation of the apparatus. Rather, whenever an exposure dosage applied to the object of irradiation has degraded to an insufficient level from repeated or prolonged operation of the apparatus, contamination of the optical components is suspected and corrective action taken.
For example, in the case of an X-ray microscope employing an LPX source, ten xe2x80x9cshotsxe2x80x9d normally are required to obtain a clear image. Whenever the number of shots required to obtain a suitably clear image increases to, say, 20 shots, the X-ray optical components of the microscope are deemed to be excessively contaminated. In X-ray microlithography apparatus, the constituent X-ray optical components are deemed contaminated when the time required to achieve transfer of a pattern having a certain minimum linewidth becomes excessively long.
In each of the foregoing methods, the presence or absence of contamination of the optical components is adjudged only after the contamination has begun to exert a large adverse influence on operation of the apparatus. In other words, the presence or absence of contamination of the optical components is unknown until the effects of contamination are manifest to an apparent unacceptable degree.
In view of the problems of the prior art, as summarized above, an object of the present invention is to provide apparatus and methods with which the state of surficial contamination of an optical component in an optical system is measured. Another object is to provide any of various optical systems, such as an X-ray microlithography apparatus, with which one or more constituent optical components can be monitored for contaminant accumulation so as to allow the need for cleaning or replacement of the optical component to be determined.
To such ends, and according to a first aspect of the invention, apparatus are provided for measuring accumulation of a contaminant substance on a surface of an optical component that, during use, is irradiated with radiation. An embodiment of such an apparatus comprises a contaminant-measuring means situated and configured to perform several tasks. First, the contaminant-measuring means detects electrons emitted by the optical component in response to the optical component being irradiated with the radiation. Of the detected electrons, the contaminant-measuring means selects electrons in a specified energy range, and obtains a measurement of the detected electrons in the specified energy range so as to obtain a measurement of a corresponding amount of accumulated contaminant substance on the optical component. An exemplary measurement is of the quantity of electrons in the specified energy range.
In this apparatus, the radiation can be electromagnetic radiation (e.g., X-rays or ultraviolet radiation) or charged-particle-beam radiation (e.g., an electron beam) sufficient to cause emission of electrons (e.g., photoelectrons and/or Auger electrons) from a surface of the optical component irradiated with the radiation.
The apparatus also can include means for irradiating the optical component. Such means can be, for example, an X-ray optical system situated and configured to direct an X-ray beam from a source to the optical component.
The contaminant-measuring means can comprise detection means for detecting electrons emitted from the optical component, wherein the detection means produces an output signal having a parameter corresponding to a detected parameter of the electrons within the specified energy range. The detected parameter can be, for example, time of flight of the electrons. Generally, the number of electrons emitted from the optical component changes (typically is reduced) whenever a contaminant substance accumulates on the optical component. Hence, the contaminant-measuring means is configured such that the state of contamination of the optical component is detected by measuring the quantity of electrons in the specified energy range.
Typically, such as in an X-ray optical system, the optical component is a reflective component that comprises a surficial material. In such an instance, the energy range can be specified based on a characteristic of the surficial material and on an energy parameter of the radiation impinging on the surficial material. Alternatively or in addition, the energy range can be specified based on a particular contaminant substance predicted to adhere to the optical component.
Further alternatively, the contaminant-measuring means can comprise a substance-identification means configured to produce a spectrum of detected electrons in the specified energy range. The spectrum, in such a configuration, can be based on the surficial material, the particular contaminant substance that could adhere to the surficial material, and on an energy parameter of the radiation. In general, the bond energy of electrons varies according to the particular irradiated substance from which the electrons are emitted. Hence, electrons emitted in response to incident radiation (e.g., electromagnetic radiation) having a particular frequency have a different energy according to the particular substance. This difference allows contaminant accumulation to be monitored readily.
The substance-identification means can be configured to identify the contaminant substance by evaluating respective positions and magnitudes of one or more spectral peaks in the spectrum. The specified energy range can include a first energy range based on a characteristic of the surficial material and on an energy parameter of the radiation, and a second energy range based on a particular contaminant substance predicted to adhere to the optical component and on the energy parameter of the radiation. In the latter configuration, the substance-identification means is configured to determine respective numbers of electrons in each of the first and second energy ranges.
The contaminant-measuring means can be configured to detect electrons in the first energy range so as to produce a first signal, and to detect electrons in the second energy range so as to produce a second signal. The contaminant-measuring means in this configuration obtains a measurement of a state of contamination of the optical component by comparing the first and second signals.
The contaminant-measuring means can comprise detection means with which electrons are detected that are emitted within a specified angular range from the optical component. The detection means produces an output signal having a parameter corresponding to a detected parameter of the electrons within the specified energy range. In such a configuration, the contaminant-measuring means can comprise means for varying a detection angle of said detection means relative to the optical component.
Generally, the distance in which electrons emitted from a substance can pass through a physical object and escape without undergoing inelastic scattering depends on the energy of the emitted electrons. Accordingly, whenever electrons are emitted in a direction perpendicular to the surface of the substance, it is possible to measure electrons that are emitted from locations deeper within the substance than otherwise possible with electrons emitted at other angles relative to the surface. Hence, the depth at which emitted electrons can be detected can be varied by varying the detection angle of the detection means.
The optical component (e.g., X-ray mirror, filter, reticle, etc.) can be provided with a surficial material that emits electrons of a specified energy whenever the optical component is irradiated. For example, if emission of electrons does not occur from an optical component of interest, the optical component can be provided with a surficial layer of a substance having a lower bond energy than the material initially on the surface of the component. Hence, the optical component now can be monitored.
Another embodiment of an apparatus for measuring accumulation of a contaminant substance on a surface of an optical component comprises a detector and a processor. The detector is situated relative to the optical component so as to receive electrons emitted by the optical component in response to the optical component being irradiated with the radiation. The detector is configured to detect the electrons emitted from the optical component and produce a corresponding output signal. The processor is connected to the detector, and is configured to select, for data processing by the processor, at least a portion of the output signal corresponding to detected electrons in a specified energy range. The processor also obtains a measurement of the detected electrons in the selected energy range. As noted above, the electrons can be photoelectrons and/or Auger electrons.
In this embodiment, the specified energy range can be determined based on a characteristic of a material from which the optical component is made and on an energy characteristic of the radiation. For example, the optical component can be a multi-layer mirror, in which instance the material is configured as an outermost layer of the mirror.
As noted with the previous embodiment summarized above, the specified energy range can be determined based on a characteristic of a contaminant substance predicted to attach to the optical component and on an energy characteristic of the radiation. The detector can comprise a substance identifier that produces a spectrum of electrons within the specified energy range. In the latter instance, the specified energy range can be determined based on a characteristic of a material from which the optical component is made, and the processor can be configured to determine peak values within the spectrum and to identify the contaminant substance adhering to the optical component from the peak values of the spectrum.
The processor can be a computer and/or can comprise discrete portions. For example, the processor can comprise a signal processor, a selector, a comparator, and a memory. The signal processor is configured to process the output signal from the detector. The selector is connected to the signal processor and is configured to determine the specified energy range. The comparator is connected to the selector and is configured to compare a detected quantity of electrons in the specified energy range to a threshold value. The memory is connected to the comparator and is configured to store data corresponding to the threshold value.
According to another aspect of the invention, methods are provided for, with respect to an optical system comprising an optical component that is irradiated with radiation during use, measuring accumulation of a contaminant substance on the irradiated surface of the optical component. In an embodiment of the method, a beam of radiation is directed to impinge on the surface. The radiation desirably is of a quality that causes the surface to emit electrons in response to being irradiated with the radiation. Electrons emitted from the irradiated surface are detected. Of the detected electrons, electrons are selected that are in a specified energy range. The detected electrons in the specified energy range are measured so as to obtain a measurement of a corresponding amount of accumulated contaminant substance on the surface. The beam comprising the radiation can be a beam of electromagnetic radiation such as X-rays or ultraviolet light, or a beam of charged particles such as an electron beam.
In the above-summarized method, the step of measuring the detected electrons can be performed first and second times, wherein the second time is later than the first time and the optical component is used between the first and second times. In such an instance, the method can further comprise the step of comparing respective measurements of electrons obtained in the first and second times.
Alternatively, the step of measuring the detected electrons can comprise comparing a detected quantity of electrons in the specified energy range to a threshold value, and determining whether the detected quantity exceeds the threshold value.
Further alternatively, the step of measuring the detected electrons can comprise producing a spectrum of detected electrons in the specified energy range, and evaluating respective positions and magnitudes of one or more spectral peaks in the spectrum. In this embodiment, electrons can be selected in both a first energy range and in a second energy range. The first energy range can be based on a characteristic of a material from which the surface is made, and the second energy range can be based on a characteristic of a contaminant substance predicted to adhere to the optical component during use. In such an instance, respective numbers of electrons in each of the first and second energy ranges are determined. The method can further include the step of comparing the respective detected numbers of electrons in the first and second energy ranges.
Yet another aspect of the invention is utilized in optical apparatus in which the surface of an object is irradiated with X-rays or other radiation capable of causing an irradiated surface to emit electrons. Specifically, in this aspect, a device is provided for monitoring accumulation of a contaminant substance on an optical component of the optical apparatus. An embodiment of the device comprises an illumination-optical system, a detector, and a processor. The illumination-optical system is situated and configured to direct a beam of radiation (e.g., X-rays) from a radiation source along a trajectory leading to the optical component. The detector is situated relative to the optical component so as to receive electrons emitted by the optical component in response to the optical component being irradiated with the beam of radiation. The detector is configured to detect the electrons emitted from the optical component and produce a corresponding output signal. The processor is connected to the detector, and is configured to select at least a portion of the output signal corresponding to detected electrons in a specified energy range. The processor also is configured to determine a measurement of the detected electrons in the selected energy range, and to determine an amount of surficial contamination of the optical component with the contaminant substance from the measurement. The overall apparatus can be any of various microlithography apparatus, for example, such as an X-ray microlithography apparatus.
The apparatus can include a projection-optical system, wherein the optical component is in the projection-optical system. In such an instance, the optical component can be, for example, an X-ray-reflective mirror.
The apparatus can include a vacuum chamber enclosing the projection-optical system. In such a configuration, the vacuum chamber typically is connected via an evacuation port to a vacuum pump (which represents a source of contamination to adjacent components). Hence, the optical component desirably would be one that is situated adjacent the evacuation port. Alternatively or in addition, a monitored optical component can be one that is situated in the projection-optical system adjacent a substrate exposed by the X-ray beam. This is because the substrate typically is coated with a resist, which also represents a source of contamination to adjacent optical components.
According to yet another aspect of the invention, methods are provided, in the context of a method for performing microlithography of a substrate with a pattern using an energy beam passing through an optical system, for measuring accumulation of a contaminant substance on a surface of an optical component of the optical system. In an embodiment of such a method, a beam of radiation is directed to impinge on the surface. The radiation is of a quality that causes the surface to emit electrons in response to being irradiated with the radiation. Electrons emitted from the surface are detected. Of the detected electrons, electrons in a specified energy range are selected. The detected electrons in the specified energy range are detected so as to obtain a measurement of a corresponding amount of accumulated contaminant substance on the surface. Hence, it is possible to perform microlithographic exposure while also monitoring the state of contamination of one or more optical components.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.